How deep are they? I'm thinking they were used by the area's pre-white man natives for grinding acorns, nuts, and other seeds into flour. Most of the time they'd be in more permnant rock outcrops, but individual boulders were also used if they had a pre-existing cavity and were in a better location. Grinding holes are usually a little bit wider, but were also shallow enough so the resulting flour could be scooped out.

These look like the kind of holes left behind after a large boulder is blasted into pieces by a dynamite. I have seen holes like these on canyon walls, rock formations next to a trail, and even on creek beds. If the explosive is not pushed all the way into the hole, what's left behind is a hole that doesn't go through the rock.

if the holes are in designated wilderness, it's likely that the gas-powered rock hammer (Pionjar being the most common manufacturer) was omitted and instead the holes were drilled by hand. Hand drilling typically involves one person swinging a sledge hammer and another person rotating a "star bit" after each hit. Sometimes one person can handle these tasks with a smaller hammer and bit. Once the holes are drilled, sometimes dynamite is used to break the rock - if permitted. More common are feathers and wedges (see pic below), which mechanically crack and break the rock into a designed shape. I used to work on a Forest Service trail crew so I love this stuff. In general it's good form to hide the evidence of rock drilling, to maintain a more natural look.

Marcsoltan wrote:These look like the kind of holes left behind after a large boulder is blasted into pieces by a dynamite. I have seen holes like these on canyon walls, rock formations next to a trail, and even on creek beds. If the explosive is not pushed all the way into the hole, what's left behind is a hole that doesn't go through the rock.

So far that seems most believable-- except it is odd that one is in a boulder with rounded edges...

Many thanks for the responses. Consider this: The holes do look like those you drill for explosives, but these were along 2 miles of trail, in small rocks, large rocks, on the side of the trail and in the middle of the trail. And this trail is a hiking trail, not an old road. It is not even for mountain bikes. We must have seen 50 holes like this in rocks. There must be many more that were full of stuff or covered up.

Definitely blast holes, probably made by a portable rock drill as others have suggested. In the eastern Sierra back before 9/11 I frequently saw trail crews having more fun than on the 4th of July drilling rocks for blasting that 2 big guys ( or maybe 1) could have easily moved off the trail with digging bars; they had lots of military surplus tetrytol. A lot of these holes were never loaded, and are still to be seen. They also pinned rocks in place with sections of 1 inch rebar in drilled holes.